1. Field of the Invention
This invention relates to overspeed protection. In particular, this invention relates to a method and apparatus for overspeed protection of a gas or steam turbine driving an electrical generator or other load from which the power consumed may rapidly drop.
2. Background Art
Generator breaker opening and other forms of rapid generator unloading can result in very high turbine shaft acceleration. Typically, a turbine will have a general speed control system, providing startup features and is made to maintain the turbine in continuous operation. Such a control system may or may not have an overspeed protection function. In addition, the turbine also typically has a dedicated overspeed protection system. When the speed control system does not operate properly, or when an upset occurs outside the ability of the speed control system to control, only the turbine overspeed protection system can prevent damage to the turbine and turbine shaft.
Traditionally, dedicated overspeed protection for gas and steam turbines was usually provided by a spring-loaded eccentric bolt (installed inside the turbine shaft) or a spring-loaded piston (installed outside the turbine shaft). Under high rotational speed conditions, either of these mechanisms was forced by centrifugal force to strike a lever providing a trip by closing the governor valves and trip valve(s), resulting in a turbine overspeed trip. Due to friction and wear, often an eccentric bolt does not work precisely and reliably. As a result, these bolts are now often replaced by an electronic overspeed trip device with electrical output acting on the lever or a spring-loaded rod or the valve itself.
The usual configuration for an electronic overspeed trip device comprises a solenoid valve which restrains the spring-loaded rod or valve when it is energized. Under normal turbine loading, this solenoid is energized. If the turbine experiences a high rotational speed, the solenoid is de-energized by the electronic overspeed trip device and the turbine trips and decelerates, perhaps shutting down entirely. Such an episode may occur immediately after an opening of the generator breaker or rapid generator unloading. A disadvantage of this solution is the high solenoid current required for spring compression for resetting the rod or valve decreases the reliability of the electronic overspeed trip device circuitry.
An unreliable solenoid power supply circuit may be the cause of false turbine trips due to insufficient current from the power supply.